The present invention relates to a compound for controlling the dielectric constant of liquid crystal and, more particularly, to a compound to be added to such a liquid crystal as a nematic liquid crystal which is used for an electrooptical display device so as to drive said display device by low driving voltage.
A typical liquid crystal display cell is the Field Effect Mode (hereinafter called as F.E.M.) cell proposed by M. Schadt et al (Applied Physics Letters, 18, 127-128 (1971). This F.E.M. cell comprises two transparent electrode plates arranged opposedly and nematic liquid crystal having positive of dielectric anistropy filled between said two electrodes, wherein longitudinal axes of molecules of said liquid crystal being parallel to said electrodes and being twisted in herical arrangement by a predetermined angle between said electrodes, thereby imparting a predetermined rotatory capacity for incident rays. When the voltage is applied on the electrode plates, these liquid molecules become vertically arranged of their longitudinal axes in respect of the electrode plates, thereby causing the rotatory capacity to disappear. This change in the rotatory capacity is converted to the change of optical transmittancy of the cell by utilizing a polarizer so as to enable displays.
However, a recent display device utilizing the F.E.M. cell requires such a relatively high driving voltage as 3 volts. Accordingly, in order to use small driving battery and to decrease the driving electric power for the display device, various kinds of method for driving the display device by a driving voltage lower than 3 volts have been researched and developed. In order to enable the low voltage driving of F.E.M. cells, the anisotropy of dielectric constant for the nematic liquid crystals to be filled in the cell is controlled to be at a certain positive value above a predetermined value, thereby lowering the threshold voltage for the said nematic liquid crystals as much as possible. For this purpose, it is preferable to use an addition compound for controlling an anisotropy of dielectric constant of liquid crystal and more particularly to utilize a large positive anisotropy of dielectric constant. By adding such a compound, it becomes possible to use the nematic liquid crystal having a negative or a comparative small anisotropy of dielectric constant for F.E.M. cells, and low voltage driving becomes also possible. In this case, the compound, which is added to the liquid crystal, for controlling the dielectric constant thereof is required essentially to have the following properties (1) to (5).
(1) The anisotropy of dielectric constant of the compound should be of an extremely large positive value;
(2) The compound dissolves well in various nematic liquid crystals, does not separate or deposit when left at low temperature and does not increase viscosity of nematic liquid crystal;
(3) The compound does not radically decrease the temperature range for the nematic liquid crystal, nor shift to the high temperature side;
(4) The compound does not disturb the ordered orientation of molecular structure of the nematic liquid crystal;
(5) The compound has excellent chemical stability, and does not decompose by water, light, heat, etc.
Many effective compounds have been known heretofore as compounds for controlling dielectric constant which include 4-substituted benzoic acid-4'-cyanophenyl ester (Japanese Patent Publication No. 52-2902), 4-substituted bensylideneamino-4'-cyano aniline (Japanese Patent Publication No. 52-942) and 4, 4'-disubstituted biphenyl (Japanese Laid Open Publication No. 49-95882). However, none of these known compounds satisfies all the properties listed above. 4-substituted benzoic acid-4'-cyanophenyl ester, for example, is inferior in respect to said property (2) and the first half of said property (3) and, at the same time, 4-substituted benzylideneamino-4'-cyano aniline is inferior in respect to said property (5), especially the property against water. Moreover, 4, 4'-disubstituted biphenyl is also inferior in respect to said properties (1) and (2).